Fiber is typically classified according to its diameter. Monofilament fiber is generally defined as having an individual fiber diameter greater than 15 denier, usually greater than 30 denier per filament. Fine denier fiber generally refers to a fiber having a diameter less than 15 denier per filament. Microdenier fiber is generally defined as fiber having less than 100 microns diameter. The fiber can also be classified by the process by which it is made, such as monofilament, continuous wound fine filament, staple or short cut fiber, spun bond, and melt blown fiber.
A variety of fibers and fabrics have been made from thermoplastics, such as polypropylene, highly branched low density polyethylene (LDPE) made typically in a high pressure polymerization process, linear heterogeneously branched polyethylene (e.g., linear low density polyethylene made using Ziegler catalysis), blends of polypropylene and linear heterogeneously branched polyethylene, blends of linear heterogeneously branched polyethylene, and ethylene/vinyl alcohol copolymers.
Of the various polymers known to be extrudable into fiber, highly branched LDPE has not been successfully melt spun into fine denier fiber. Linear heterogeneously branched polyethylene has been made into monofilament, as described in U.S. Pat. No. 4,076,698 (Anderson et al.), the disclosure of which is incorporated herein by reference. Linear heterogeneously branched polyethylene has also been successfully made into fine denier fiber, as disclosed in U.S. Pat. No. 4,644,045 (Fowells), U.S. Pat. No. 4,830,907 (Sawyer et al.), U.S. Pat. No. 4,909,975 (Sawyer et al.) and. in U.S. Pat. No. 4,578,414 (Sawyer et al.), the disclosures of which are incorporated herein by reference. Blends of such heterogeneously branched polyethylene have also been successfully made into fine denier fiber and fabrics, as disclosed in U.S. Pat. No. 4,842,922 (Krupp et al.), U.S. Pat. No. 4,990,204 (Krupp et al.) and U.S. Pat. No. 5,112,686 (Krupp et al.), the disclosures of which are all incorporated herein by reference. U.S. Pat. No. 5,068,141 (Kubo et al.) also discloses making nonwoven fabrics from continuous heat bonded filaments of certain heterogeneously branched LLDPE having specified heats of fusion. While the use of blends of heterogeneously branched polymers produces improved fabric, the polymers are more difficult to spin without fiber breaks and/or dripping at the spinneret die.
U.S. Pat. Nos. 5,294,492 and 5,593,768 (Gessner), both incorporated herein by reference, describe a multiconstituent fiber having improved thermal bonding characteristics composed of a blend of at least two different thermoplastic polymers which form a continuous polymer phase and at least one noncontinuous polymer phase. In the claims, Gessner recites that the at least one noncontinuous phase occupies a substantial portion of the surface of the fiber made from the blend. But while we believe the claims in U.S. Pat. Nos. 5,294,492 and 5,593,768 specify, for example, a core-sheath configuration with respect to the polymer phases, the photomicrograph (FIG. 1 therein) shows an island-sea type phase configuration for the fiber cross-section. Further, we believe it is the continuous polymer phase (not the noncontinuous phase) which occupies a substantial portion of the surface of the fiber exemplified (but not claimed) by Gessner. Also, all of the Examples (and presumably FIG. 1 therein) consist of polypropylene polymer blended with ASPUN.TM. fiber grade LLDPE resins having a 12 or 26 g/10 minute I.sub.2 melt index as supplied by The Dow Chemical Company. The exemplar polypropylene polymer used by Gessner was described a "controlled rheology" PP (i.e. a visbroken PP) having a melt flow rate of 26 and at least 90 percent by weight isotacticity.
U.S. Pat. No. 5,549,867 (Gessner et al.), incorporated herein by reference, describes the addition of a low molecular weight (i.e. high melt index or melt flow) polyolefin to a polyolefin with a molecular weight (M.sub.z) of from 400,000 to 580,000 to improve spinning. The Examples set forth in Gessner et al. are all directed to blends of 10 to 30 weight percent of a lower molecular weight metallocene polypropylene with from 70 to 90 weight percent of a higher molecular weight polypropylene produced using a Ziegler-Natta catalyst.
U.S. Pat. No. 4,839,228 (Jezic et al.), incorporated herein by reference, describes biconstituent fibers having improved tenacity and hand composed of a highly crystalline polypropylene polymer with LDPE, HDPE or preferably LLDPE. The polyethylene resins are described to have a moderately high molecular weight wherein their I.sub.2 melt index is in the range of from about 12 to about 120 g/10 minutes.
Also, fibers made from blends of visbroken polypropylene polymer and homopolymer high density polyethylene (HDPE) having an I.sub.2 melt index of equal to greater than 5 g/10 minutes are known. Such blends are thought to function on the basis of the immiscibility of the olefin polymers.
WO 95/32091 (Stahl et al.) discloses a reduction in bonding temperatures by utilizing blends of fibers produced from polypropylene resins having different melting points and produced by different fiber manufacturing processes, e.g., meltblown and spunbond fibers. Stahl et al. claims a fiber comprising a blend of an isotactic propylene copolymer with a higher melting thermoplastic polymer.
WO 96/23838, U.S. Pat. Nos. 5,539,056 and 5,516,848, the disclosures of which are incorporated herein by reference, teach blends of an amorphous poly-.alpha.-olefin of Mw&gt;150,000 (produced via single site catalysis) and a crystalline poly-.alpha.-olefin with Mw&lt;300,000, (produced via single site catalysis) in which the molecular weight of the amorphous polypropylene is greater than the molecular weight of the crystalline polypropylene. Preferred blends are described to comprise about 10 to about 90 weight percent of amorphous polypropylene. The described blends are said to exhibit unusual elastomeric properties, namely an improved balance of mechanical strength and rubber recovery properties.
U.S. Pat. No. 5,483,002 and EP 643100, the disclosures of both of which are incorporated herein by reference, teach blends of a semi-crystalline propylene homopolymer having a melting point of 125 to 165.degree. C. and a semi-crystalline propylene homopolymer having a melting point below 130.degree. C. or a non-crystallizing propylene homopolymer having a glass transition temperature which is less than or equal to -10.degree. C. These blends are said to have improved mechanical properties, notably impact strength.
Crystalline polypropylenes produced by single site catalysis have been reported to be particularly suited for fiber production. Due to narrow molecular weight distributions and low amorphous contents, higher spinning rates and higher tenacities have been reported. But, isotactic PP fibers, in general (and particularly when produced using single site catalyst) exhibit poor bonding performance.
U.S. Pat. No. 5,677,383 (Lai et al.), incorporated herein by reference, discloses blends of (A) at least one homogeneously branched ethylene polymer having a high slope of strain hardening coefficient and (B) at least one ethylene polymer having a high polymer density and some amount of a linear high density polymer fraction. The Examples set forth by Lai et al. are directed to substantially linear ethylene interpolymers blended with heterogeneously branched ethylene polymers. Lai et al. describe the use of their blends in a variety of end use applications, including fibers. The disclosed compositions preferably comprise a substantially linear ethylene polymer having a density of at least 0.89 grams/centimeters.sup.3. But Lai et al. disclosed fabrication temperatures only above 165.degree. C. In contrast, to preserve fiber integrity, fabrics are frequently bonded at temperatures less than 165.degree. C. such that all of the crystalline material is not melted before or during the fiber bonding step.
While various olefin polymer compositions have found success in a number of fiber and fabric applications, the fibers made from such compositions would benefit from an improvement in bond strength, which would lead to stronger fabrics, and accordingly to increased value to the nonwoven fabric and article manufacturers, as well as to the ultimate consumer. But any benefit in bond strength must not be at the cost of a detrimental reduction in spinnability and fiber elongation nor a detrimental increase in the sticking of the fibers or fabric to equipment during processing.